Wireless communication systems provide various types of communication such as voice, data, video and the like. These systems are based on different modulations techniques such as code division multiple access (CDMA), time division multiple access (TDMA), etc. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards.
Pseudorandom noise (PN) sequences are commonly used in CDMA systems for spreading transmitted data, including transmitted pilot signals. The time required to transmit a single value of the PN sequence is known as a chip, and the rate at which the chips vary is known as the chip rate. CDMA receivers commonly employ RAKE receivers. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilots from one or more base stations, and two or more multipath demodulators (fingers) for receiving and combining information signals from those base stations.
Inherent in the design of direct sequence CDMA systems is the requirement that a receiver must align its PN sequences to those of a base station. Some systems, such as those defined by the W-CDMA standard, differentiate base stations using a unique PN code for each, known as a primary scrambling code. The W-CDMA standard defines two Gold code sequences for scrambling the downlink, one for the in-phase component (I) and another for the quadrature (Q). The I and Q PN sequences together are broadcast throughout the cell without data modulation. This broadcast is referred to as the common pilot channel (CPICH). The PN sequences generated are truncated to a length of 38,400 chips. The period of 38,400 chips is referred to as a radio frame. Each radio frame is divided into 15 equal sections referred to as slots. W-CDMA base stations operate asynchronously in relation to each other, so knowledge of the frame timing of one base station does not translate into knowledge of the frame timing of any other base station. In order to acquire this knowledge, W-CDMA systems require synchronization channels and a cell searching technique.
Referring now to FIG. 1, a chart illustrates the various synchronization channels utilized in a W-CDMA system to perform synchronization. These channels include a Primary Synchronization Channel (Primary SCH) which is coded with a Primary Synchronization Code (PSC). The purpose of the PSC is to provide slot timing. There is also a Secondary Synchronization Channel (Secondary SCH) which is coded with one of 16 possible Secondary Synchronization Codes (SSC) per slot. A length 15-SSC sequence identifies the code group and frame timing. There is also the Common Pilot channel (CPICH) which is scrambled with primary scrambling codes. The CPICH is utilized to obtain the primary scrambling code. From FIG. 1, it can be appreciated that Primary SCH and the Secondary SCH are divided into 15 slots numbered Slot 0 through Slot 14, and that each slot is 2560 chips. Both the PSC and the SSC are 256 chips longs. There is also a Primary Common Control Physical Channel (PCCPCH) within a cell, and it is used to carry synchronization and broadcast information for users. The Primary CCPCH is not transmitted during the first 256 chips of each slot. Instead, the Primary SCH and Secondary SCH are transmitted during this period, the remainder being used for broadcast messages.
It is possible to search for W-CDMA base stations offset by offset (38,400 of them) for each of the 512 primary codes. However, this is not practical due to the excessive amount of time such a search would require. Instead, the W-CDMA standard calls for base stations to transmit the Primary SCH and the Secondary SCH, to assist the mobile terminal in searching efficiently. As a result, a W-CDMA cell search can be performed in three steps.
For initial acquisition, the three-step W-CDMA search technique provides a substantial performance increase, in terms of reduced search time, over the impractical alternative of searching the entire PN space for each scrambling code. Probability of detection and search time are important metrics in determining the quality of a CDMA system. Decreased search time or higher probability of detection implies that searches can be done faster or less often. As such, a subscriber unit can locate and access the best available cell faster or more reliably using less power, resulting in better signal transmission and reception, often at reduced transmission power levels by both the base station and the subscriber unit. This, in turn, increases the capacity of the CDMA system (either in terms of support for an increased number of users, or higher transmission rates, or both). Furthermore, decreased search time is also advantageous when a subscriber unit is in idle mode, a low-power state where a subscriber unit is not actively transmitting or receiving voice or data, but is periodically monitoring the system. Reduced search time allows the subscriber unit to spend more time in the low power state, thus reducing power consumption and increasing standby time.
In a conventional three step cell searching technique, step2 is performed using only the SSC signal. One such cell searching technique is described in U.S. Pat. No. 6,768,768, by Rao, et al., entitled “Method and apparatus for step two W-CDMA searching”, which is incorporated herein by reference in its entirety and which assigned to the assignee of the present application. The '768 patent discloses several embodiments for improving the second searching step. In one embodiment of the '768 patent, a plurality of codes or SSCs, are correlated with a received signal at a plurality of offsets to produce a code/slot energy corresponding to each code/slot boundary pair. Unique subsets of the code/slot energies are summed to produce code sequence energies, the maximum of which indicates a located code sequence and slot boundary. In another embodiment of the '768 patent, the correlation is performed by sub-correlating the received signal with a common sequence, and performing a Fast Hadamard Transform (FHT) on the results. In yet another embodiment, one sub-correlator can be used to search a plurality of peaks simultaneously.
Referring now to FIG. 2, a block diagram illustrates the hardware of the Rao '768 patent for performing one of the conventional step2 search algorithms. FIG. 1 depicts a searcher 430 having I and Q samples which enter correlator 510, where they are correlated with each of the sixteen SSCs. The correlator 510 contains sub-correlator 520, FHT 530, and energy calculator 535. Sub-correlator 520 produces a length-16 sub-correlation sequence for delivery to FHT 530. The results of the correlator 510 are stored in memory 540. The energy results for multiple frames may be accumulated and stored in memory 540. Summer 550 reads SSC/slot energy values out of memory 540 according to a predetermined SSC sequence for each slot hypothesis. The SSC/slot energies are added to produce a SSC sequence energy. The SSC sequence energies are delivered to maximum energy detector 560 for detection of the maximum energy, which corresponds to the most likely scrambling code group and frame timing. Summer 550 and maximum energy detector 560 may be combined in one circuit, or the functions of both can be carried out in a DSP.
Another cell searching technique is described in an article by Y.-P. E Wang, T. Ottosson, which is entitled “Cell search in W-CDMA”, IEEE Journal on Selected Areas in Communications, Vol 18, Issue 8, August 2000, page 1470-1482, which is incorporated herein in its entirety. In this technique, the authors proposed a coherent detection method, using the phase of PSC correlation to correct the phase of SSC correlation. The idea is that the random phase φk can be estimated using PSC correlation. This estimated phase can then be used to phase-correct the SSC correlation.
W-CDMA searchers designed to reduce search time will improve the speed and performance of the mobile terminal. In addition, however, efficiency of implementation is also important to reduce integrated circuit area and power consumption. Step2 of the three step search method described above is a complex procedure, and there is therefore a need in the art for efficient searchers that can perform step2 W-CDMA searching.